Optical loss in a glass fiber is the measure of the purity of the glass, and describes how the light is attenuated from the input end of the fiber to its output end. The lower the loss, the greater the distance that light can travel before it must be amplified. Loss through glass is particularly low in the wavelength region 1200-1600 nm, and yet for years lightwave transmission has been confined to the wavelength regions around 1310 nm and 1550 nm. A number of factors have conspired to confine transmission to these regions including: fiber bending loss above 1600 nm; the gain characteristic of present-day optical amplifiers; Rayleigh scattering; and hydroxyl-ion (OH) absorption centered around 1385 nm. With regard to the availability of light sources in the 1360-1430 nm wavelength region, a "no-man's land" has been created. However, there is no physical barrier to producing optical sources throughout the wavelength range 1200-1600 nm with the Indium Phosphide (InP)--based materials system. In fact, many researchers have produced lasers at various wavelengths in this region precisely to study optical absorption not only in fiber, but also in the characterization of atmospheric contaminants. Moreover, fiber-amplifier pump lasers have been made to emit at 1480 nm.
FIG. 1 shows the overall spectral loss curve for an optical fiber having a glass core. The loss curve is shown in the wavelength region where the overall loss is low enough for practical optical systems to operate. Loss in this wavelength region is primarily attributable to Rayleigh scattering and OH absorption.
Rayleigh scattering is a basic phenomenon that results from density and compositional variations within the fiber material. These variations occur when the glass is produced, since it must pass through the glass transition point in becoming an amorphous solid. There is a certain level of thermal agitation occurring at the transition point, causing thermal and compositional fluctuations that are "frozen" into the lattice at the softening point and are dependent on material composition. The scale of these imperfections is smaller than the wavelength of the light. They are fundamental, cannot be eliminated, and set the lower limit on fiber loss. Rayleigh scattering is proportional to 1.lambda..sup.4, where .lambda. is the wavelength of the light.
Optical loss at 1385 nm is a measure of the water remaining in the glass. The more water that is present, the higher the loss. Accordingly, hydroxyl-ion absorption is frequently referred to as "water" absorption, and it arises from lightwave energy being absorbed by the OH ion at wavelengths that are related to its different vibration modes. For example, the two fundamental vibrations of this ion occur at 2730 nm and 6250 nm and correspond to its stretching and bending motions respectively. Nevertheless, overtones and combination vibrations strongly influence the loss in the near infrared and visible wavelength regions. In particular, the overtone at 1385 nm resides in the heart of a transmission region where future optical fiber systems may be operated. It has long been desirable to reduce this particular "water peak" to as low a value as possible. Unfortunately, concentrations of OH as low as one part per million (ppm) cause losses as high as 65 dB/km at 1385 nm. And while it is desirable to reduce OH concentration to a level such that the overall optical loss at 1385 nm is comparable to the overall optical loss at 1310 nm (i.e., about 0.33 dB/km), it has not been commercially feasible to reduce it a thousandfold to about 0.8 parts per billion (ppb). Such an OH concentration would add 0.05 dB/km to the Rayleigh scattering loss at 1385 nm in order for the overall loss at this wavelength to be about 0.33 dB/km.
Three "windows" are shown in FIG. 1--each identifying a wavelength region for normal operation on an optical fiber. Historically, early fiber systems operated near 825 nm (the first window) because laser sources and detectors became available at these wavelengths in 1979. Second window systems operating near 1310 nm became available between 1980 and 1983 and, more recently, third window systems operating near 1550 nm were introduced in 1986. For future optical systems, elimination of the water peak at 1385 nm, in a commercially available optical fiber, would effectively open the entire wavelength range 1200-1600 nm for lightwave transmission.
In multimode fibers, lightwaves are strongly confined to the core due to the relatively large difference in refractive index between the core and the deposited cladding that surrounds it. And since lightwaves are effectively confined to the core in multimode fibers, OH ions in the cladding do not have a significant effect on optical loss. Indeed, multimode fibers having low OH absorption in the 1385 nm region have been fabricated and are reported in the literature. See, for example, Moriyama et al. Ultimately Low OH Content V.A.D. Optical Fibres, Electronics Letters, Aug. 28, 1980 Vol. 16, No. 18, pp. 698-699. However, it is desirable to fabricate a singlemode fiber, wherein a significant portion of the energy travels in the cladding, having a low water-absorption peak at 1385 nm.
A singlemode optical fiber having a low water-absorption peak at 1385 nm was reported during August, 1986 in the article Recent Developments in Vapor Phase Axial Deposition by H. Murata, Journal of Lightwave Technology, Vol. LT-4, No. 8, pp. 1026-1033. However, low water absorption is achieved by initially depositing a substantial amount of cladding onto the core prior to overcladding with a silica tube. (The VAD process is capital intensive, and any reduction in productivity increases the manufacturing cost to the point that depositing large amounts of cladding are unacceptable for the mass production of preforms.) A figure of merit (D/d), known as the deposited cladding/core ratio, has been defined as the ratio of the diameter of the rod (D) to the diameter of the core (d); and it is desirable for this dimensionless number to be as low as possible because the amount of deposited material is proportional to (D/d).sup.2. Murata reports that the deposited cladding/core ratio is greater than 7.5 before it is overclad with a silica-tube in order to assure low OH content in the fiber for a number of different overcladding tubes. Nevertheless, it is desirable to fabricate a core rod having low OH content wherein D/d is less than 7.5.
It is known to fabricate an optical fiber having low OH content using the modified chemical vapor deposition (MCVD) process such as shown in U.S. Pat. No. 5,397,372 that issued on Mar. 14, 1995. In this patent, a hydrogen-free plasma torch is used for the deposition of high-index material inside a glass tube. The glass tube is then collapsed to become a preform, but only short lengths of fiber (e.g., 0.7 km) can be drawn from such a preform. In commercial production, however, large preforms are required for making long lengths of fiber. And the rod-in-tube technique is a cost-effective way of making large preforms, although OH contamination can be a serious problem.
Accordingly, what is sought is an optical transmission system that is capable of operating over long distances at wavelengths in the 1360-1430 nm region. More importantly, what is sought a singlemode optical fiber having a low water peak at 1385 nm and a commercially viable process for making same.